Corrosion Resistance of CeO2-GO/Epoxy Nanocomposite Coating in Simulated Seawater and Concrete Pore Solutions

Reinforced concrete structures in the marine environment face serious corrosion risks. Coating protection and adding corrosion inhibitors are the most economical and effective methods. In this study, a nano-composite anti-corrosion filler with a mass ratio of CeO2:GO = 4:1 was prepared by hydrothermally growing cerium oxide on the surface of graphene oxide. The filler was mixed with pure epoxy resin at a mass fraction of 0.5% to prepare a nano-composite epoxy coating. The basic properties of the prepared coating were evaluated from the aspects of surface hardness, adhesion grade, and anti-corrosion performance on Q235 low carbon steel subjected to simulated seawater and simulated concrete pore solutions. Results showed that after 90 days of service, the corrosion current density of the nanocomposite coating mixed with corrosion inhibitor was the lowest (Icorr = 1.001 × 10−9 A/cm2), and the protection efficiency was up to 99.92%. This study provides a theoretical foundation for solving the corrosion problem of Q235 low carbon steel in the marine environment.


Introduction
For industrial development and market demand, steel-reinforced concrete is widely applied as an engineering material in the construction of a range of offshore infrastructure such as ports and coastal, confederation bridges, and transport pipelines, which are increasingly being put into production and operation [1]. However, seawater corrosion can lead to problems such as surface cracking, declining load carrying capacity, and lower life expectancy of concrete-reinforcing bars [2][3][4][5], and steel corrosion poses a serious risk to economic safety [6]. For this reason, the study on corrosion protection of concretereinforcing bars under marine environment can minimize economic losses and promote the development of marine resources [7][8][9]. Reinforcing bar corrosion in the marine environment is one of the major challenges, and it is therefore of great significance to study its protection measures.
Traditional corrosion protection methods such as concrete surface treatment, coating protection, electrochemical cathodic protection of metals, and the addition of corrosion inhibitors can inhibit the corrosion of concrete-reinforcing bars [10][11][12]. Of these methods mentioned above, coating protection is favored due to its high corrosion protection efficiency, simplicity of process, and low cost [13,14]. Among all kinds of coating anticorrosion technologies, epoxy coatings are of great interest due to their excellent working application of this composite material and provide a new solution for the corrosion protection of concrete buildings. Compared with the pure epoxy resin, The results shows that the hardness and adhesion of CeO 2 :GO (4:1) nanocomposite coating increased by nearly 50% compared with pure epoxy coating after 30 days of service. After 60 days of service, the corrosion degree of steel bar decreased significantly, and the corrosion area decreased from 41.4% to 22.8%. The electrochemical impedance value increased by three orders of magnitude. After 90 days of service, the corrosion current density of the nanocomposite coating mixed with corrosion inhibitor was the lowest (I corr = 1.001 × 10 −9 A/cm 2 ), and the protection efficiency was up to 99.92%.

Raw Materials
Graphene oxide was supplied by Changzhou Sixth Element Materials Technology Co., Changzhou, China. Cerium nitrate hexahydrate was purchased from Shanghai Aladdin Biochemical Technology Co., Shanghai, China. Epoxy resin (WSR6101 E-44) and epoxy AB adhesive were supplied by Nantong Xingchen Synthetic Materials Co, Nantong, China. The epoxy resin (WSR6101 E-44) is a bisphenol thermosetting epoxy resin with a viscosity of 15,000 mPa·s at 25 • C. The selected Lanxing Lan-826 multifunctional pickling corrosion inhibitor was purchased from Henan Xinyang Chemical Co, Xinyang, Henan Province, China. CaCl 2 , NaF, KCl, NaHCO 3 , KBr, H 3 BO 3 , NaCl, and Na 2 SO 4 were supplied by Chengdu Kolon Chemical Co., Chengdu, China. Table 1 describes the chemical composition of Q235 steel sheet. The basic properties of Lan-826 corrosion inhibitor are shown in Table 2.

Preparation of CeO 2 -GO (4:1)/EP Nanocomposite Coating
CeO 2 -GO nanocomposites were prepared by hydrothermal synthesis method and mixed with epoxy resin to prepare CeO 2 -GO/EP composite coatings. In 500 mL of deionized water, 10 g of sodium bicarbonate and 186.6 mg of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA.2Na) were dissolved. The solution was stirred well and placed in a dialysis bag. The solution in the dialysis bag was boiled for 10 min. After the dialysis bag was cleaned, the graphene oxide liquid was added, and the mixture was boiled again in a beaker for 10 min. Following that, the liquid was dissolved in deionized water and dispersed by ultrasonic treatment for 30 min. The dispersed solution was transferred into the dialysis bag and stirred in a beaker for three days. Finally, the liquid was extracted from the dialysis bag and a graphene oxide dispersion was obtained using an ultrasonic cell disrupter. 0.807 g of cerium nitrate hexahydrate was weighed and dissolved in a beaker containing 4 mL of ammonia solution. The solution was stirred with a glass rod and then transferred into a polytetrafluoroethylene-lined container, which was placed inside a high-pressure reactor. The reactor was placed in an oven set at 160 • C for 24 h. After the reaction was complete, the product was filtered, dried, and ground to obtain CeO 2 -GO nanoparticles. In the experiments, Q235 steel sheets(150 mm × 70 mm × 1 mm) were used to assess the hardness and adhesion of the coating, and Q235 mild steel with dimensions of 5 mm in height and 10 mm in diameter was selected to evaluate the anticorrosive properties of the coating. The prepared CeO 2 -GO composite material was mixed with epoxy resin in a mass ratio of 0.5% and coated on the surface of Q235 steel through a wire rod coating device. After the coating solidified, it was rubbed with sandpaper to ensure that the coating thickness was controlled at 100 µm by a coating thickness gauge. The samples were immersed in simulated seawater and concrete pore liquid composite solutions for 90 days to test the coating's corrosion resistance.
For comparison, as shown in Table 3, four epoxy coating samples, G1, G2, G3, and G4, were prepared, with G1 being the pure epoxy coating, G2 being the epoxy coating with corrosion inhibitor, G3 being the CeO 2 -GO/EP coating, and G4 being the CeO 2 -GO/EP coating with corrosion inhibitor. The anti-corrosion performance of the four coatings subjected to simulated seawater and simulated concrete pore liquid composite solutions were evaluated.

Preparation of Corrosive Solutions
Different concentrations of simulated seawater and simulated concrete pore liquid composite solutions (20%, 40%, 60%) were prepared according to ASTM D1141-1998 (2013). As shown in Figure 1, 24.53 g of NaCl and 4.09 g of Na 2 SO 4 were dissolved in 800 mL of the aqueous solution. We slowly added 20 mL of stock solution No. 1 with vigorous stirring, then added 10 mL of stock solution No. 2. The solution pH was adjusted to 12.5 with Ca(OH) 2 after dilution to 1 L. Table 4 shows the chemical composition of the simulated seawater and simulated concrete pore solution.    To visualize the microstructure of CeO 2 -GO nanocomposite coatings in the seawater and concrete pore liquid environments with long-term service, a scanning electron microscope (FESEM, Gemini, M/s.Zeiss, Karlsruhe, Germany) was used with a working distance of 7 mm and an acceleration voltage of 1 kV at room temperature. With pure epoxy coating as a contrast, G1 and G3 samples were immersed in 60% hybrid solution for 60 days to characterize the micromorphology of the corroded coating substrate and to evaluate the anti-corrosion effect of CeO 2 -GO (4:1) nanocomposites. When calculating the erosion area of the sample, we selected the observation area and determined its size and shape. The size of the observed area can be measured using the eyepiece scale and objective scale of a microscope. Based on the measured shape and size, the area of the corroded part of the sample can be calculated.

Hardness Test
The hardness of the coatings was tested according to standard ISO 15184:1998 [37]. The hardness of a coating was determined using the pencil scratch hardness test. The hardness of the hardest pencil that does not leave a scratch exceeding 3 mm on the coating was used to represent the hardness of the coating. The higher the hardness of the coating, the better the wear resistance of the coating. The coating samples were evenly coated with (20 ± 3) µm thickness on a Q235 steel surface of 150 mm × 70 mm × 1 mm, respectively. After the samples were cured at room temperature for 24 h, the hardness of the coatings were measured at a temperature of (23 ± 2) • C and relative humidity of (50 ± 5)%. All samples were immersed in different concentrations of the solutions simultaneously for 15 days to obtain the hardness of the coatings after service in the simulated seawater and simulated concrete pore liquid composite solutions.
The adhesion values of the coatings were determined according to standard ISO 2409:2013 [38]. The sample preparation is the same as the hardness test. The coating samples were evenly coated with (20 ± 3) µm thickness on a carbon steel surface of 150 mm × 70 mm × 1 mm, respectively. After each sample was immersed in the solution for 30 days, the coating adhesion tester was used to conduct the test under the conditions of temperature (23 ± 2) • C and relative humidity (50 ± 5)%. First, the cured coating template was fixed in a position perpendicular to a grid device, and a cross-shaped scratch was made on it. Then, a transparent tape was uniformly covered over the scratch. Finally, the tape was removed, and the adhesion strength was determined by observing whether the scratch comes off or not according to certain criteria. As shown in Table 5, the adhesion grade was divided into six grades according to the area of the coating surface peeling off. The smaller the grade, the smaller the area of the coating peeling off, and the better the adhesion.

Electrochemical Test
Electrochemical tests were performed using a CHI-760E electrochemical workstation with a three-electrode system. The platinum electrode and saturated glycury electrode were used as counter electrode and reference electrode, respectively. G1, G2, G3, and G4 were conducted as working electrodes after the open circuit potential values were stabilized. The exposed area of the working electrode is 1 square centimeter. In electrochemical experiments, the thickness of all coatings is controlled at 100 µm. In EIS testing, the initial potential (Init E (V)) is the stable value of the open circuit potential, the high frequency (Hz) is 100,000, the low frequency (Hz) is 0.01, the amplitude (V) is 0.005, and the quiet time (sec) is 2.
After the samples were immersed in the composite solution for 90 days, the tafel polarization curves were recorded at a scanning speed of 0.5 mV/s in the range of −200 mV to +1200 mV to investigate the trends of kinetic parameters of the coatings after long-term service in simulated seawater and simulated concrete pore liquid composite solutions with different concentrations. Table 6 describes the time when all electrochemical tests were done. The EIS and Tafel tests were completed simultaneously within 90 days. In the EIS testing, as there was no significant difference between the 90 day test results and the 60 day test results, in order to avoid data redundancy, Section 3.4.2 only analyzed the EIS test results within 60 days.

SEM-EDS Analysis of CeO 2 -GO Nanocomposites
Figures 2 and 3 show the SEM corrosion morphologies of G1 and G3 after 60 days of service in a 60% corrosive solution. According to Figure 2a, the surface of the epoxy coating is cracked. The pure EP coating has a severe degree of corrosion, and there are deep corrosion pits due to large-scale peeling of the coating. From Figure 2b-d, it can be seen that G1 shows a large area of rough and concave corrosion pits, and a large number of corrosion products exist on the surface. The corrosion area of the substrate was observed through an optical microscope, with the black shaded area being the rusted portion and the white area being the uncorroded portion. Figures 2e and 3e show the macroscopic morphology of G1 and G3 after 60 days of corrosion, respectively. Calculations revealed that the erosion portion of G3 was 22.8%, which was only half that of the G1. As shown in Figure 3b-d, the uniform corrosion of G3 coating occurred because the graphene oxide modified by cerium oxide was uniformly distributed in the epoxy resin in the form of lamellae, filling the microporous defects of the pure epoxy resin. The scanning electron microscopy and energy spectrum analysis results of CeO 2 -GO (4:1) ( Figure 4) revealed that CeO 2 was successfully grafted onto the surface of GO, indicating the successful preparation of CeO 2 -GO (4:1) nanocomposite material. Figure 3b-d, the uniform corrosion of G3 coating occurred because the graphene oxide modified by cerium oxide was uniformly distributed in the epoxy resin in the form of lamellae, filling the microporous defects of the pure epoxy resin. The scanning electron microscopy and energy spectrum analysis results of CeO2-GO (4:1) ( Figure 4) revealed that CeO2 was successfully grafted onto the surface of GO, indicating the successful preparation of CeO2-GO (4:1) nanocomposite material.      Figure 5 shows the hardness test results for all coatings after 30 days of immersion in different concentrations of the simulated mixture. The hardness value of the coating doped with CeO2-GO nanocomposite material was nearly twice that of the pure EP coating, which indicates that the graphene oxide with excellent mechanical properties filled the microporous defects in the pure epoxy resin and improved the hardness of the pure  Figure 5 shows the hardness test results for all coatings after 30 days of immersion in different concentrations of the simulated mixture. The hardness value of the coating doped with CeO 2 -GO nanocomposite material was nearly twice that of the pure EP coating, which indicates that the graphene oxide with excellent mechanical properties filled the microporous defects in the pure epoxy resin and improved the hardness of the pure epoxy resin [39]. As shown in Figure 6, as the concentration increases, the hardness values of G1 and G2 decreased significantly, while that of G3 and G4 decreased slightly. The hardness values of G1, G2, G3, and G4 coatings were in descending order of magnitude. The reason is that there were many bubbles and pores on the surface and inside of the pure EP coating, and more corrosion solutions immersed in the coating, causing the surface of the coating to expand and crack. The hardness value of the same sample decreased with the increase of solution concentration, demonstrating that the higher the concentration of the solution, the more quickly the corrosive medium penetrated the coating, and the faster the hardness of the coating decreased. As can be seen in Figure 6, the addition of corrosion inhibitors can slightly delay the rate of decline in the hardness value of the coating, but its effect was not apparent.

Hardness
Polymers 2023, 15, x FOR PEER REVIEW 9 of 21 epoxy resin [39]. As shown in Figure 6, as the concentration increases, the hardness values of G1 and G2 decreased significantly, while that of G3 and G4 decreased slightly. The hardness values of G1, G2, G3, and G4 coatings were in descending order of magnitude. The reason is that there were many bubbles and pores on the surface and inside of the pure EP coating, and more corrosion solutions immersed in the coating, causing the surface of the coating to expand and crack. The hardness value of the same sample decreased with the increase of solution concentration, demonstrating that the higher the concentration of the solution, the more quickly the corrosive medium penetrated the coating, and the faster the hardness of the coating decreased. As can be seen in Figure 6, the addition of corrosion inhibitors can slightly delay the rate of decline in the hardness value of the coating, but its effect was not apparent.

Adhesive Force
The results of the adhesion test after 30 days of serviced in the simulated seawater/concrete pore liquid composite solution environment are presented in Figure 6. G3 and G4 had the lowest adhesion grade and the smallest spalling area, indicating that CeO2-GO (4:1) nanocomposites are fully bonded with epoxy resin and CeO2-GO/EP has good stability. Compared with CeO2-GO/EP coating, pure EP coating has poor crack resistance due to the loose and porous surface, which makes the corrosion ions penetrate continuously. After 30 days of service, the adhesion grades of G1 and G2 decreased significantly, while the adhesion of G3 and G4 coatings was still larger. As shown in Figure 7, after 30 days of service, the adhesion values of G1 and G2 coatings decreased significantly, while that of G3 and G4 decreased slightly. It can be seen that CeO2-GO (4:1) nanocomposites effectively improved the adhesion of the coating, which was consistent with the hardness test results.

Adhesive Force
The results of the adhesion test after 30 days of serviced in the simulated seawater/concrete pore liquid composite solution environment are presented in Figure 6. G3 and G4 had the lowest adhesion grade and the smallest spalling area, indicating that CeO 2 -GO (4:1) nanocomposites are fully bonded with epoxy resin and CeO 2 -GO/EP has good stability. Compared with CeO 2 -GO/EP coating, pure EP coating has poor crack resistance due to the loose and porous surface, which makes the corrosion ions penetrate continuously. After 30 days of service, the adhesion grades of G1 and G2 decreased significantly, while the adhesion of G3 and G4 coatings was still larger. As shown in Figure 7, after 30 days of service, the adhesion values of G1 and G2 coatings decreased significantly, while that of G3 and G4 decreased slightly. It can be seen that CeO 2 -GO (4:1) nanocomposites effectively improved the adhesion of the coating, which was consistent with the hardness test results.  Spectroscopy   Figures 8 and 9 show the Nyquist diagrams of each coating during 60 days of service in simulated seawater and concrete pore liquid composite solutions with different concentrations. Figure 8 describes the Nyquist diagrams of coatings immersed in composite solutions with different concentrations (20%, 40%, 60%) for 30 days. Each coating showed a large capacitive arc in the low-frequency zone, indicating that the coating is in the early immersion stage. At this time, the epoxy resin was well isolated from the corrosive ions in the composite solution due to its strong adhesion ability and good electrical insulation. It is worth noting that, as shown in Figure 8c, in the 60% concentration composite solution, G1 presented two capacitive arcs, which indicates that the pure EP coating has entered the middle stage of corrosion. The corrosion medium has gradually penetrated the substrate surface through the pores and defects of the coating. As shown in Figure 8f, after the coating had served in the 60% solution for 30 days, the second half capacitive arc appeared in G2 and G3, and G1 completely formed two capacitive arcs. Although the Nyquist diameter of the G4 reduced, it still appeared as a capacitive arc. It can be found that the G4 coating was in the initial stage of corrosion, and other coatings were in the middle stage of corrosion. Figure 9 shows the Nyquist diagrams of each coating immersed in the hybrid solution for 60 days, and the capacitive arc radius of the curve decreased regularly. Figure 9a shows the Nyquist diagrams of the coatings after 45 days of service in a 20% composite solution. The first capacitive arc of G1 and G2 further decreased, and G3 coating presented a complete capacitive arc and a second capacitive arc, which indicates that G3 began to form defects. At this time, G4 still showed good corrosion resistance due to the inhibition of corrosion inhibitors. In Figure 9b, the capacitive arc radius of G1, G2, G3, and G4 presented two arcs from small to large, which indicates that all coatings began to enter the middle stage of corrosion after 45 days of service in a 40% hybrid solution. Although the capacitive arc radius of the coatings still decreased in the later period of immersion, the rate decreased. It can be seen from Figure 9e that, after 60 days of service in a 40% hybrid solution environment, G1 appeared Warburg tail in the low-frequency zone, beginning to fail. As the corrosion medium continued to diffuse to the interior of the substrate, G2, G3, and G4 showed a capacitive arc. The G2, G3, and G4 all show a capacitive arc due to the passive film and corrosion products generated in the concrete pore liquid environment, delaying the metal corrosion rate. the middle stage of corrosion. The corrosion medium has gradually penetrated the substrate surface through the pores and defects of the coating. As shown in Figure 8f, after the coating had served in the 60% solution for 30 days, the second half capacitive arc appeared in G2 and G3, and G1 completely formed two capacitive arcs. Although the Nyquist diameter of the G4 reduced, it still appeared as a capacitive arc. It can be found that the G4 coating was in the initial stage of corrosion, and other coatings were in the middle stage of corrosion.   Figure 9 shows the Nyquist diagrams of each coating immersed in the hybrid solution for 60 days, and the capacitive arc radius of the curve decreased regularly. Figure 9a shows the Nyquist diagrams of the coatings after 45 days of service in a 20% composite solution. The first capacitive arc of G1 and G2 further decreased, and G3 coating presented a complete capacitive arc and a second capacitive arc, which indicates that G3 began to form defects. At this time, G4 still showed good corrosion resistance due to the inhibition of corrosion inhibitors. In Figure 9b, the capacitive arc radius of G1, G2, G3, and G4 pre- The Bode diagrams of coatings (G1, G2, G3, G4), immersed in simulated seawater and simulated concrete pore liquid-composite solution for 60 days, are shown in Figures 10 and 11. It can be seen from Figure 12 that the impedance value of each coating in different solutions was considerable when soaked for 15 days, and the impedance modulus values in the low-frequency region of all coatings show the same rule: G4 > G3 > G2 > G1. After soaking in 20% composite solution for 15 days, the low-frequency impedance modulus of G4 coating exceeds 10 8 Ω cm 2 , and the low-frequency impedance modulus of each coating decreases with the increase of corrosion solution concentration. Figure 10a,c show that the low-frequency impedance modulus of G1 and G2 decreased significantly with the increase of seawater concentration. When the solution concentration increases from 20% to 60%, the low-frequency impedance modes of G1 and G2 decrease from 10 7 Ω cm 2 to 10 6 Ω cm 2 , while the low-frequency impedance modes of G3 and G4 coatings decrease slightly. This phenomenon indicated that CeO 2 -GO/EP inhibited the corrosion rate of metals. As the immersion time increased, the corrosion resistance of the coating gradually degraded. As shown in Figures 10e and 11b, the low-frequency impedance modulus of G2 and G1 decreased from 10 6 Ω cm 2 to 10 5 Ω cm 2 after immersion in 40% hybrid solution for 30 d and 45 d. It can be seen from Figure 11 that after 60 days of service, the impedance values of all coatings continue to decrease, but the decrease is slight. The reason is that the coatings almost fail in the later stage of corrosion, which is consistent with the analysis results of the Nyquist diagram. slightly. This phenomenon indicated that CeO2-GO/EP inhibited the corrosion rate of metals. As the immersion time increased, the corrosion resistance of the coating gradually degraded. As shown in Figures 10e and 11b, the low-frequency impedance modulus of G2 and G1 decreased from 10 6 Ω cm 2 to 10 5 Ω cm 2 after immersion in 40% hybrid solution for 30 d and 45 d. It can be seen from Figure 11 that after 60 days of service, the impedance values of all coatings continue to decrease, but the decrease is slight. The reason is that the coatings almost fail in the later stage of corrosion, which is consistent with the analysis results of the Nyquist diagram.

Tafel Curve Analysis
The Tafel curve represents the strongly polarized part of the polarization curve. By studying the relationship between corrosion current density and corrosion voltage, the polarization resistance and protection efficiency of anti-corrosion coatings were analyzed [40]. The expression is as follows.
η % = 1− I corr I corr0 ×100% (2) β a represents the slope of the Tafel curve of the electrode anode. β c represents the slope of the Tafel curve of the electrode cathode. I corr represents the current corrosion density of the anti-corrosion coating. I corr0 represents the current corrosion density of a pure epoxy resin coating. R p is the coating polarization resistance of the prepared electrode. η represents the coating protection efficiency.      Figure 12 shows the Tafel curves of the coating soaked in simulated seawater and simulated concrete pore-liquid/composite solution for 90 days. Table 7 shows the corrosion kinetic parameters of the coating soaked in a 20% concentration hybrid solution for 90 days. The corrosion current density of G2 is 2.143 × 10 −7 A/cm 2 , which is smaller than that of EP coating. The corrosion current density of G4 is only 1.001 × 10 −9 A/cm 2 , slightly smaller than that of G3, and its protection rate reaches 99.92%. In comparison with G1, the corrosion current density of G4 was reduced by three orders of magnitude. The polarization resistance of G4 was the largest, which was 9.473 × 10 8 ohm, which was slightly larger than that of G3. The corrosion inhibitor could boost the erosion resistance of the metal, but the effect is not apparent. After 90 d in the 20% concentration compound solution, the relationship between the magnitude of corrosion current density is G4 < G3 < G2 < G1, while the relationship between the magnitude of corrosion potential of the coating shows the opposite rule: G4 > G3 > G2 > G1, which shows that CeO 2 -GO nanomaterials can effectively improve the corrosion protection performance of the coating. According to Tables 8 and 9, the corrosion current density of the coating increased with the ascension of solution concentration. Meanwhile, the corrosion potential and protection efficiency decreased. However, after immersion in 60% hybrid solution for 90 days, the protection efficiency of G4 is still as high as 94.76%. Compared with G1, the protection efficiency of G4 increased by 33.48%. It is proved again that CeO 2 -GO/EP can effectively improve the corrosion resistance of metal in the seawater and concrete pore liquid composite solutions environment.   The protection efficiency of the coatings after 90 d of service in pure simulated seawater and simulated seawater and simulated concrete pore liquid composite solutions, respectively, is shown in Figure 13. In the same concentration of the corrosive solution, the protection efficiency of the coating in the composite solution environment is more effective than that in the pure simulated seawater environment. It is worth noting that the self-corrosion current density of pure EP in the composite solution environment is much lower than that in the pure simulated seawater environment, indicating that the coating has better corrosion protection under concrete pore liquid conditions. water and simulated seawater and simulated concrete pore liquid composite solutions, respectively, is shown in Figure 13. In the same concentration of the corrosive solution, the protection efficiency of the coating in the composite solution environment is more effective than that in the pure simulated seawater environment. It is worth noting that the self-corrosion current density of pure EP in the composite solution environment is much lower than that in the pure simulated seawater environment, indicating that the coating has better corrosion protection under concrete pore liquid conditions.

Discussion
The graphene oxide nanocomposites modified by CeO2 can be fully dispersed in the epoxy resin, filling the microporous defects of pure epoxy resin and improving the mechanical properties of the coating. Compared with the pure epoxy resin, the results show that the hardness and adhesion of CeO2:GO (4:1) nanocomposite coating increased by nearly 50% compared with pure epoxy coating after 30 days of service. After 60 days of service, the corrosion degree of steel bar decreased significantly, and the corrosion area decreased from 41.4% to 22.8%. The electrochemical impedance value increased by three orders of magnitude. After 90 days of service, the corrosion current density of the nanocomposite coating mixed with corrosion inhibitor is the lowest (Icorr = 1.001 × 10 −9 A/cm 2 ), and the protection efficiency is up to 99.92%. On the one hand, the physical barrier effect of CeO2-GO nanocomposite dramatically improves the corrosion resistance of the coating. On the other hand, hydrates formed by hydrolysis of cerium ions also inhibit the corrosion

Discussion
The graphene oxide nanocomposites modified by CeO 2 can be fully dispersed in the epoxy resin, filling the microporous defects of pure epoxy resin and improving the mechanical properties of the coating. Compared with the pure epoxy resin, the results show that the hardness and adhesion of CeO 2 :GO (4:1) nanocomposite coating increased by nearly 50% compared with pure epoxy coating after 30 days of service. After 60 days of service, the corrosion degree of steel bar decreased significantly, and the corrosion area decreased from 41.4% to 22.8%. The electrochemical impedance value increased by three orders of magnitude. After 90 days of service, the corrosion current density of the nanocomposite coating mixed with corrosion inhibitor is the lowest (I corr = 1.001 × 10 −9 A/cm 2 ), and the protection efficiency is up to 99.92%. On the one hand, the physical barrier effect of CeO 2 -GO nanocomposite dramatically improves the corrosion resistance of the coating. On the other hand, hydrates formed by hydrolysis of cerium ions also inhibit the corrosion reaction. The schematic diagram of the coating corrosion process is shown in Figure 14a. The loose and porous surface of the pure EP coating allows for the continuous penetration of corrosion ions. As a result, the internal water absorption of the pure epoxy coating becomes more prominent, and thus the corrosion rate is faster. The CeO 2 -GO/epoxy nanocomposite coating possesses self-repair capability. This mechanism is based on the interaction between two key materials: cerium oxide and graphene oxide. When the surface of the coating sustains minor damage, the nanoparticles of CeO 2 and GO can diffuse and fill the defects and cracks, reacting with the surrounding substrate to form a new protective layer [41]. In this way, CeO 2 -GO can partially repair itself when subjected to corrosion through the interaction between cerium ions and graphene oxide, thereby maintaining its anti-corrosion properties. This "corrosion self-repair coordination mechanism" provides more reliable and long-lasting protection for anti-corrosion materials, enabling them to be used in more severe environments [42]. the coating sustains minor damage, the nanoparticles of CeO2 and GO can diffuse and fill the defects and cracks, reacting with the surrounding substrate to form a new protective layer [41]. In this way, CeO2-GO can partially repair itself when subjected to corrosion through the interaction between cerium ions and graphene oxide, thereby maintaining its anti-corrosion properties. This "corrosion self-repair coordination mechanism" provides more reliable and long-lasting protection for anti-corrosion materials, enabling them to be used in more severe environments [42].  Figure 14b is a schematic diagram of the corrosion process of CeO2-GO/EP. Cerium oxide is adsorbed on the lamellar graphene oxide surface by hydrothermal synthesis, making graphene oxide nanosheets uniformly dispersed in the epoxy coating and filling microporous defects. As the coating enters the middle and later stages of corrosion, as shown in Figure 15, the corrosion solution penetrates the coating and contacts the substrate. Meanwhile, Ce 4+ can be rapidly released into the corrosion solution due to the cation exchange mechanism [43]. Cerium oxide hydrolyzes in the corroded cathode area, producing a water-soluble precipitate layer that covers the substrate surface and inhibits corrosion in the cathode area. Additionally, as the concrete pore liquid is alkaline, the passive film is formed on the surface of the substrate, which cooperates with the corrosion products to jointly resist the further corrosion of corrosion ions.  Figure 14b is a schematic diagram of the corrosion process of CeO 2 -GO/EP. Cerium oxide is adsorbed on the lamellar graphene oxide surface by hydrothermal synthesis, making graphene oxide nanosheets uniformly dispersed in the epoxy coating and filling microporous defects. As the coating enters the middle and later stages of corrosion, as shown in Figure 15, the corrosion solution penetrates the coating and contacts the substrate. Meanwhile, Ce 4+ can be rapidly released into the corrosion solution due to the cation exchange mechanism [43]. Cerium oxide hydrolyzes in the corroded cathode area, producing a water-soluble precipitate layer that covers the substrate surface and inhibits corrosion in the cathode area. Additionally, as the concrete pore liquid is alkaline, the passive film is formed on the surface of the substrate, which cooperates with the corrosion products to jointly resist the further corrosion of corrosion ions.

Conclusions
In this paper, CeO2:GO (4:1) nanocomposites were produced by hydrothermal synthesis, and CeO2-GO/EP coatings were formulated by being mixed with epoxy resin. This paper investigates the corrosion behavior of steel reinforcement in offshore structures under different concentrations of marine environments. The study involves introducing simulated concrete pore liquid into simulated seawater to simulate the actual corrosive conditions. The corrosion resistance and mechanical properties of CeO2-GO/EP in simulated seawater/concrete pore liquid composite solutions with different concentrations were investigated. The corrosion areas of the coatings after 60 d of service were calculated by optical microscopy, and the corrosion area of CeO2:GO (4:1)/EP was reduced by half compared to that of pure epoxy (41.4%), with a value of 22.8%. The evolution of the corrosion state of the coatings was investigated by electrochemical tests during 90 d, indicating that

Conclusions
In this paper, CeO 2 :GO (4:1) nanocomposites were produced by hydrothermal synthesis, and CeO 2 -GO/EP coatings were formulated by being mixed with epoxy resin. This paper investigates the corrosion behavior of steel reinforcement in offshore structures under different concentrations of marine environments. The study involves introducing simulated concrete pore liquid into simulated seawater to simulate the actual corrosive conditions. The corrosion resistance and mechanical properties of CeO 2 -GO/EP in simulated seawater/concrete pore liquid composite solutions with different concentrations were investigated. The corrosion areas of the coatings after 60 d of service were calculated by optical microscopy, and the corrosion area of CeO 2 :GO (4:1)/EP was reduced by half compared to that of pure epoxy (41.4%), with a value of 22.8%. The evolution of the corrosion state of the coatings was investigated by electrochemical tests during 90 d, indicating that the lowfrequency impedance value of CeO 2 :GO (4:1)/EP doped with corrosion inhibitor was the largest (|Z|0.01 = 10 8 ohm cm 2 ). By calculating Tafe polarization data, the corrosion current density of CeO 2 :GO (4:1)/EP after 90 days of service was I corr = 1.435 × 10 −8 A/cm 2 . After calculation and analysis, the protection efficiency of CeO 2 :GO (4:1)/EP was 98.91%, significantly improving the corrosion resistance of the coating.